If a flight controller or an autopilot (Automatic Flight Control System, AFCS) is used, these artificial breakout forces are used to support the input of AFCS actuator signals into the aircraft control system. The forces that can be applied to the control system by such an autopilot are thus limited by these artificial breakout forces of the control system.
Overcoming the artificial breakout force or movement in the region of the force gradient is generally assessed by the AFCS computer as being an intended intervention by the pilot in the control, and thus results in temporary degradation of the AFCS operating mode in order to prevent the pilot and the AFCS from working against each other. Depending on the type of aircraft and the current flight situation, degradation of the AFCS operating mode can take place to a different extent in this arrangement. For example complete degradation of the autopilot is imaginable so that control of the aircraft takes place exclusively as a result of the manual control intervention of the pilot. However, partial degradation of the autopilot is also imaginable, in which the aircraft continues to be stabilised by the autopilot, while the pilot with manual control intervention handles the coarse control input, for example the flight direction.
This design is associated with an advantage in that it provides the pilot with a direct intervention option in the control and in unequivocal automatic degradation of the AFCS operating mode in favour of manual control by the pilot. Consequently situations can be prevented in which the AFCS and the pilot unintentionally work against each other, which might otherwise lead to critical flight situations. However, such a direct intervention option for the pilot requires moderate breakout forces in order to make comfortable manual control possible. The entire breakout forces are composed of the component friction in the control system and the additional artificial breakout forces to support the autopilot. Since the entire breakout forces are to be overcome by the pilot, there is a disadvantage in that in the case of substantial undesirable component friction it is only possible to select slight artificial breakout forces to support the autopilot, and consequently this result in high sensitivity of the system to any unintended intervention by the pilot. This can result in unintended bumping against the control device, or unintended fixing of the control device by the pilot possibly resulting in degradation of the autopilot, without this being intended and/or noticed by the pilot. Furthermore, as has already been mentioned, slight artificial breakout forces also limit the forces that can be exerted on the control system by the autopilot in the AFCS mode.
In other designs the AFCS remains active until such time as it is degraded by the pilot with the use of a switch or a contact sensor on the control device. This design advantageously results in improved robustness to unintended pilot intervention, but this advantage is counterbalanced by a disadvantage in that the absence of automatic degradation during pilot intervention, which as has already been mentioned may lead to critical flight situations. Activating the switch or the contact sensor on the control device generally results in decoupling of the trim coupling, as a result of which a trim motor, by way of which the AFCS intervenes in the control system, is decoupled from the control system. Such decoupling generally also results in a reduction in the breakout forces, e.g. by partial or complete decoupling of the artificial breakout forces. The pilot is then in a position to carry out manual control of the aircraft with reduced breakout forces. Furthermore, it is possible to use increased artificial breakout forces in the AFCS mode and thus to increase the range of forces for the autopilot because manual control by the pilot takes place with partial or complete decoupling of these artificial breakout forces. However, decoupling of the trim motor generally also results in the loss of the original trim point, which is often perceived by the pilot as being disagreeable because after termination of manual intervention the trim point has to be set anew.
Furthermore, combined embodiments exist in which degradation of the autopilot can take place both by way of direct intervention by the pilot in the control system by overcoming the entire breakout forces and by activation of a switch or of a contact sensor on the control device. These embodiments are, however, also associated with the already mentioned disadvantages in that the artificial breakout forces that are available to the autopilot for acting on the control system need to be moderate in order to make manual intervention by the pilot possible, and in that during decoupling of the trim coupling the trim point is lost.